Organic materials provide exciting prospects for applications in electronic devices including, for example, printed electronics, solar cells, light-emitting diodes, and thin film transistors. In particular, solar cells (or photovoltaic devices (PVs)) are important due to a growing economic need for a practical source of renewable energy that will substantially reduce dependence upon fossil fuels. Silicon-based solar energy systems have been touted for years as potential candidates. However, the capital-intensive nature of silicon manufacturing processes contributes to a cost structure that falls significantly short of commercial viability. Photovoltaic cells, or solar cells, based on Inherently Conductive (or generally used, Conducting) Polymers (ICPs) (such as polyacetylene, polythiophene, polyaniline, polypyrrole, polyfluorene, polyphenylene, polycarbazole, and poly(phenylene vinylene) offer a great potential as significantly lower-cost devices because these polymers can be handled like inks in conventional printing processes.
Alternative sources of energy, especially renewable energy, are being sought to dramatically change the functional and cost boundaries resulting from current energy sources. This need is heightened by the rapidly increasing cost, environmental impact, and geo-political implications of the world's reliance on fossil fuels. Regulations from the global level (e.g., Kyoto Treaty) to the local level increase the demand for cost-effective renewable energy supply. The use of the sun's rays to create power represents an attractive, zero-emission source of renewable energy.
Conjugated polymers are a key component of a new generation of organic solar cells (or organic photovoltaics (OPVs)) that promises to significantly reduce the cost/performance barrier of existing inorganic counterparts. The primary advantage of an organic solar cell is that the core materials, and the device itself, present flexible, light-weight design advantages and can be manufactured on an industrial scale in a low-cost manner. Organic components can be solution processed and printed by standard printing techniques to form thin films. However, while this technology holds great promise, commercialization hurdles remain. There is a great demand for materials with a fine balance of processability, stability, electronic and spectroscopic properties (e.g., conductivity, charge transport, band gap, energy spacing between the HOMO and LUMO levels (highest occupied and lowest unoccupied molecular orbitals, respectively) that would substantially improve OPV performance.
Among the multitude of conducting polymers investigated to date, polythiophene (PT) and its derivatives continue to represent a versatile conjugated polymer system. This is largely due to their exceptional spectroscopic and electronic properties, potential ease of processing, relative robustness, and light weight. In order to influence the material properties in a desired fashion, it is of key importance to structurally control the molecular organization and molecular composition of the conjugated polymers. Extensive studies have been done with the poly(3-alkylsubstituted thiophene) (PAT) system. The initial synthetic approaches for making PATs had virtually no control over their absolute structures. Due to the presence of configurational isomers, the polymers possessed various degrees of regioregularity. The synthetic methodologies that afford regioselective synthesis of PATs are based on transition metal promoted cross-coupling reaction of organometallic compounds and halide derivatives of β-functionalized thiophenes. The scope of this type of metal-assisted cross coupling polymerizations has been expanded enormously by the development of efficient initiators or catalysts. For example, the degree of regioregularity has been shown to be controlled and affected by the ligands' influence on the metal center and the choice of the metal (Chen et al., J. Am. Chem. Soc. 1995, 117, 233). Both nickel and palladium metal complexes with tailored phosphine ligands have been applied.
For the alkyl-substituted polythiophenes (e.g., poly(3-hexylthiophene) [P3HT]), with certain specifications, reaction requirements and conditions, the Grignard Metatheses (GRIM) and McCullough methodologies for the dibromo-functionalized starting monomers afford regioregular polymer. However, if the reaction conditions are altered or other thiophene derivatives (e.g., aryl-substituted thiophenes and their functionalized counterparts) are used, the regioregularity of the final polymer may not be easily controlled, despite attempted adherence to the classic GRIM and McCullough protocols. Therefore, there is a need for an improved method to achieve, for example, reproducible and structurally pure poly(3-aryl substituted thiophene) products. Furthermore, for scaling up processes for PATs (including P3HT) it becomes of a challenge to control regioregularity with some methodologies (e.g., the GRIM method applied for the industrial production of P3HT). As a result, a better methodology is needed.